ChemGlobe: Argon - Electronic, thermal, and steric data along with basic information. - http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/18.html
LANL: Argon - Sources, properties, isotopes, and uses. - http://periodic.lanl.gov/elements/18.html
Wikipedia: Argon - Details the properties of the element, including its history, applications, and characteristics. - http://en.wikipedia.org/wiki/Argon
Lenntech: Argon - Physical data, chemical properties, health and environmental effects. - http://www.lenntech.com/Periodic-chart-elements/Ar-en.htm
EnvironmentalChemistry.com: Argon - Atomic structure, chemical and physical properties, and table of nuclides. - http://environmentalchemistry.com/yogi/periodic/Ar.html
Visual Elements: Argon - General and physical information, uses, key isotopes, and ionisation energies. - http://www.chemsoc.org/viselements/pages/argon.html
ChemicalElements.com: Argon - Basic information, atomic structure, and table of isotopes. - http://www.chemicalelements.com/elements/ar.html